Flexible dampers for attachment between a rotor hub assembly and a rotor blade or cuff which exhibit a high degree of damping are desirable for damping lead-lag motions of rotor blades on helicopters. Typically, elastomers are used to provide this damping, such as taught in U.S. Pat. No. 5,092,738 to Byrnes et al., U.S. Pat. No. 4,778,343 to Hahn et al., and U.S. Pat. No. 4,893,988 to Sato. The high degree of damping exhibited by these dampers is typically produced by bonding the dampers in a highly-damped elastomer such as silicone. However, these elastomeric dampers are inherently nonlinear as a function of amplitude and can produce a loss factor of only about 0.7. This is insufficient, under certain conditions, to control excessive motions of the helicopter rotor blade.
Additional damping can be added, over and above what is available in an elastomeric damper, by incorporating a fluid within the damper to provide damping at certain predetermined, or tuned, frequencies. The commonly assigned U.S. Pat. No. 4,811,919 to Jones, which is hereby incorporated by reference, describes the addition of fluid to an elastomeric mounting which is suitable for mounting an aircraft engine to a strut, and also describes the means for compensating for the fluid expansion due to temperature changes. U.S. Pat. No. 4,236,607 to Halwes et al. describes a vibration suppression system wherein the fluid passes through the inner member of the mounting to provide amplified counter inertial forces.
U.S. Pat. No. 4,566,677 to Le Pierres which is herein incorporated by reference describes a damped fluid-and-elastomeric, lead-lag damper for helicopter rotor blades, whereby the fluid is used to add additional damping over and above what is available from the elastomer alone. However, this damper is axially-acting and cannot carrying vertical loads due to flapping, which must be accommodated in bearingless rotor systems. Furthermore, U.S. Pat. No. 5,004,215 to Aubry et al., which is also herein incorporated by reference, describes another axially acting strut-type damper with linear damping by incorporating a viscous fluid. Again, this device cannot carry vertical loads due to flapping motions of the rotor blade, a normally required feature of dampers which are to be used in bearingless rotor systems.
In rotor-damping systems, and in fluid devices in general, the requirements for high damping, which necessitates the use of damped elastomers and viscous fluids, tend to make the dampers undesirably stiff when exposed to low temperatures. The Aubry et al. '215 device describes a resilient return strut for use with helicopter rotors which uses reed-type valves for limiting the pressure which can build up within the fluid chambers. However, these types of pressure-limiting valves do not allow for free flow of fluid during normal operating conditions. U.S. Pat. No. 4,613,118 to Morita describes a fluid mounting suitable for connecting to an engine which incorporates a valve mechanism 450 to limit the pressure within the fluid chamber. Upon encountering a situation which requires pressure relief, a threshold pressure is exceeded, and more flow area is opened in fluid-parallel relation with the fluid orifice. This mechanism is complicated, non-linear, in that it has different properties in each direction, and requires a separate orifice and valve in parallel in order to get unimpeded or unblocked flow for normal operating conditions. Alternate embodiments in Morita allow for the valve and fluid-flow path to be in a fluid-series relationship. However, this arrangement does not allow for unblocked flow during normal operating conditions.